Gradle: Compile Gradle models/model builders under JVM 1.6

This prevents imports failure in projects using older versions of Gradle

 #KT-21610 Fixed

idea/kotlin-gradle-tooling/build.gradle.kts

@@ -3,6 +3,8 @@ description = "Kotlin Gradle Tooling support"
 
 apply { plugin("kotlin") }
 
+jvmTarget = "1.6"
+
 dependencies {
     compile(projectDist(":kotlin-stdlib"))
     compile(project(":compiler:cli-common"))

plugins/android-extensions/android-extensions-idea/build.gradle.kts

@@ -3,6 +3,8 @@ description = "Kotlin Android Extensions IDEA"
 
 apply { plugin("kotlin") }
 
+jvmTarget = "1.6"
+
 dependencies {
     compile(project(":compiler:util"))
     compile(project(":compiler:light-classes"))

plugins/kapt3/kapt3-idea/build.gradle.kts

@@ -1,6 +1,8 @@
 
 apply { plugin("kotlin") }
 
+jvmTarget = "1.6"
+
 dependencies {
     compile(projectDist(":kotlin-stdlib"))
     compile(project(":compiler:frontend"))